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Mark of Calth

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Targetdrone.gif This article is about the Mark; for the Anthology, see Mark of Calth (Anthology).

The term Mark of Calth refers to two related phenomena occurring in the course of the Battle of Calth, during the Horus Heresy.

Battle Counter

The first use refers to the Ultramarines' official time index for the progress of the battle. It was customary for the Ultramarines to commence a timer at the outset of any engagement, so that events could be put into context for later analysis.[1b]

Mark Zero of the Battle of Calth commenced when Roboute Guilliman reluctantly issued orders allowing his Space Marines to return fire against the Word Bearers, if doing so was necessary to defend themselves.[1a][1e] Although Mark Zero was reckoned the official "start" of the battle, it occurred approximately 136 hours after the Word Bearers had seized the fleet tender Campanile in the outskirts of the Calth system[1a] and twenty minutes after the Word Bearers commenced their attack on the Ultramarines.[1c][1d]

The Mark of Calth continued to run after the cessation of immediate hostilities between the Ultramarines and the Word Bearers in the Calth system. The Ultramarines' destruction of the Word Bearers homeworld of Colchis was reckoned an extension of the Battle of Calth, occurring at Mark 219,479.25.03. The Mark continued to run through the Horus Heresy and the ten thousand years after. It is still running, and will continue to do so until the Ultramarines have exterminated the entire Word Bearers Legion and their Primarch, Lorgar[1h].

The "Mark" of Calth

During the Battle, the Word Bearers seized control of the orbital weapons platforms around Calth and turned their weapons onto the Veridian star, Calth's sun.[1f] They intended to induce a catastrophic supernova, but the Ultramarines prevented this. Instead, the star was poisoned and began to bombard Calth with lethal radiation. The survivors of the combat were forced to retreat into Calth's massive cave system, where the Battle continued as the Underworld War.[1g]

Many human civilians and soldiers suffered terrible radiation burns, which remained with them for the rest of their lives. Although these burns were painful and unsightly, the survivors of the Battle stubbornly refused to have them surgically removed, declaring that the "Mark of Calth" was a badge of honour.[1b]


  • 1: Know No Fear (Novel)
    • 1a: Target/Acquisition, Chapter 1
    • 1b: Target/Acquisition, Chapter 2
    • 1c: Target//Acquisition, Chapter 8
    • 1d: Absolute//Overwhelm, Chapter 2
    • 1e: Absolute//Overwhelm, Chapter 3
    • 1f: Ushkul//Thu, Chapter 2
    • 1g: Ruin//Storm, Chapter 4
    • 1h: Un//Doing, Epilogue